The present invention relates to rotors for caliper disc brakes and the like, and in particular to a rotor having a friction surface machined by an electric discharge grinding machine and a method for making the same.
Rotors are generally well known in the art, and are used extensively in vehicle braking systems. Vehicle caliper disc braking systems slow the vehicle by inhibiting the rotation of the vehicle wheels. Rotors used in typical vehicle braking systems include a central hat section for attaching the rotor to a vehicle wheel and drive member for rotation therewith, and an outer friction section having opposite friction surfaces.
A caliper assembly is secured to a non-rotating component of the vehicle, such as the vehicle frame. The caliper assembly includes a pair of brake pads disposed adjacent the rotor friction surfaces, and a moveable piston operatively connected to one or more of the brake pads. When the driver brakes the vehicle, hydraulic or pneumatic forces move the piston which clamps the pads against the friction surfaces of the rotating rotor. As the brake pads press against the moving rotor friction surfaces, frictional forces are created which oppose the rotation of the wheels and slow the vehicle. The friction converts the vehicle's kinetic energy into large quantities of heat, much of which is absorbed by the friction surfaces and conducted to the rest of the rotor and to other components to which the rotor is connected.
Brake rotors are typically cast from a ferrous material, such as cast or grey iron. The rotors are then machined in multiple operations to shape the rotor, to form the inner mounting section and friction surfaces. However, ferrous material rotors are relatively heavy and they corrode during normal use. Brake rotors are also cast from aluminum based metal matrix composite (MMC) containing silicon carbide particulate reinforcement. Aluminum MMC rotors have sufficient mechanical and thermal properties at a significantly reduced weight compared to ferrous metal rotors. Typically, the rotor is cast from aluminum MMC and then machined in a conventional manner to form the finished rotor.
During the manufacture of the rotor, the friction surfaces of the rotor are machined to a predetermined tolerance range. Conventional machining techniques for rotors use physical contact between the friction surface and a tool to achieve a finish machined surface. For example, the friction surfaces are machined by grinding the friction surfaces against one ore more grinding wheels, or by turning the friction surfaces on a lathe against one or more cutting tools.
A continuous method for machining the friction surfaces of the rotor utilizes a CNC (computer numerical control) lathe. The CNC lathe includes a pair of cutters located on opposite sides of the rotor and initially positioned at either an inward or outward radial direction (depending on their initial position) relative to the axis of the rotor as the rotor is turned. While appearing to produce substantially flat surfaces, the cutters actually operate to machine spiral grooves in each of the friction surfaces of the rotor.
It also known to machine the friction surfaces by using an "interrupted" turning method. According to this method, the cutters machine in a radial direction relative to the axis of the rotor to a predetermined distance, at which point the cutters dwell for a predetermined time while the rotor continues to rotate. This creates a single groove extending circumferentially around the friction surfaces of the rotor a full 360.degree.. The cutters continue machining in the radial direction until the next predetermined distance is reached, at which point the cutters dwell for a single rotation of the rotor to form another separate groove. This pattern is repeated throughout the machining operation to produce progressively smaller or larger circumferential grooves. These type of grooves may also be referred to as concentric grooves.
Other machining techniques, such grinding, are known to form grooves on the surface of a rotor which do not extend a full 360.degree. around the circumference of the rotor. The groove may extend along only a portion the circumference of the friction surface of the rotor. As the groove extends along the portion of the circumference of the friction surface of the rotor, the groove may also extend inwardly or outwardly in the radial direction. It is also known to grind the friction surfaces of the rotor in both the clockwise and the counterclockwise directions forming grooves that intersect each other.
The friction surface of a rotor may also be machined by roller burnishing. Roller burnishing is a cold-working process that uses pressure rolling techniques to manipulate the surface material of a work piece. In the case of a rotor, the roller burnishing may be used as a finish machining step to reduce the size of the peaks of the grooves on the friction surface of the rotor created by other conventional machining operations. However, roller burnishing does not remove all of the grooves created during the prior rough machining step.
The grooves or directional markings on the friction surface of the rotor produces undesirable results. The grooves cause the brake pads of the caliper assembly to move in the radial direction on the braking surface of the rotor during braking. The combination of the grooves in the friction surface of the rotor and the movement of the brake pads manifest themselves as "clacking" or other undesirable noise, pedal pulsations. Additionally, grooves in the braking surface can cause non-uniform wear of the pads and the braking surface.
It is desirable to provide a brake rotor having braking surfaces which are machined to predetermined tolerances without having grooves or directional markings.